Like most new parents, my husband and I couldn’t get over how exquisitely tiny our newborn baby was. Baby V’s toes were smaller than Tic Tacs, her fingernails were the size of red pepper flakes and her eyelashes were almost imperceptible. I remember nestling her entire bent arm in the “L” formed by my thumb and pointer finger.

So it was really strange when we took one-month-old Baby V to meet a friend’s one-week-old baby. Our daughter appeared to have grown inches in seconds. Next to the newborn, Baby V seemed colossal.

It turns out that a similar illusion is at work in most families. When asked in a survey, more than 70 percent of mothers said that after the birth of their second child, the previous baby of the family suddenly appeared bigger, Jordy Kaufman of the Swinburne University of Technology in Australia and colleagues report December 16 in Current Biology.

The reason for this Alice in Wonderland growth spurt has little to do with the kid who suddenly seems big. Instead, parents routinely—and substantially—underestimate the size of their youngest. In the study, the researchers asked 77 mothers to estimate the height of their (out of sight) kids by marking a wall. The kids were later measured precisely.

Moms were good at sizing up their older kids. But for the youngest children, the mothers failed abysmally, underestimating the youngest kids’ height by a whopping 7.5 centimeters, the team found. And moms with one child shorted their kids by about the same amount. Age didn’t seem to matter, either. Kids from age 2 to 6 all fell short in their mothers’ eyes.

The researchers call this size underestimate the “baby illusion,” an inaccuracy that might make sense evolutionarily. By seeming smaller and therefore more vulnerable than the other kids, the youngest one might be coddled more, the researchers write. This illusion is shattered by the arrival of a newer, needier kid. When the jig is up, parents finally see the elder child at his or her actual size — and to many parents, that size is surprisingly big.

I wonder whether the illusion ever disappears if no younger sibling arrives to challenge it. Are youngest children (or only children) forever doomed to being 3 inches too short in their parents’ minds? Note to self: Ask Mom to estimate the height of my very tall 31-year-old younger brother. But really, the number won’t matter. Even though he’s a grown man who towers over us at Christmas, he will always be the baby of the family.